1. Technical Field
This disclosure generally relates to liquid ejection head units and image forming apparatuses and, in particular, to a liquid ejection head unit and an image forming apparatus capable of detecting liquid droplets ejected from a liquid ejection head.
2. Description of the Related Art
General image forming apparatuses, such as printers, facsimile machines, copiers, plotters, or multi-task machines having plural functions thereof, include a recording head composed of liquid ejection heads that eject liquid droplets of ink so as to form images. During image formation (used synonymously with recording, printing, and imaging), the image forming apparatuses cause the liquid droplets of the ink to adhere to a sheet, while transferring a medium (hereinafter referred also to as a “sheet,” but it does not limit a material. Also, it is used synonymously with a print medium, a medium to be recorded, a recording medium, a transfer member, a recording paper, etc.).
Note that the “image forming apparatus” refers to an apparatus that ejects liquid onto a medium such as a paper, a thread, a fiber, a fabric, leather, metal, a plastic, glass, wood, and a ceramic so as to perform the image formation. Furthermore, the “image formation” refers to forming on the medium not only meaningful images such as characters and graphics, but also meaningless images such as patterns (merely, ejection of liquid droplets). Furthermore, the “ink” is not limited to narrowly-defined ink and is not particularly limited so long as it turns into liquid when ejected. For example, the ink refers also to a DNA sample, a resist, a pattern material, a resin material, etc.
In such a liquid-ejection-type image forming apparatus, liquid droplets are ejected from fine pores called nozzles of the liquid ejection head, thereby making it possible to form patterns on the medium in a non-contact manner. Therefore, the image formation is made possible in a single image forming process regardless of the types and shapes of the medium.
Each of the nozzles has a fine orifice having a diameter of 50 μm or smaller and ejects liquid droplets from the orifice. Therefore, it is very difficult to maintain the nozzle in a normal state. Particularly, in the case of a line-type head having nozzle arrays in which nozzles having a width equivalent to that of the medium are arranged side by side, there are an enormous number of (several hundred through several thousand) nozzles. Therefore, it is a significant problem to maintain all the nozzles in a normal state, while avoiding ejection failures, such as a case in which the liquid droplets cannot be ejected from the nozzles, a case in which the liquid droplets are not ejected in the direction approximately perpendicular to a nozzle surface, and a case in which a desired size of the liquid droplets cannot be ejected.
Thus, it is necessary to identify an ejection failure nozzle and recover its operation so as to maintain the nozzle in a normal state. Conventionally, in a personal ink jet printer, etc., a nozzle check pattern is printed on a sheet so as to visually confirm the states of nozzles. However, this method wastefully consumes a sheet and requires time for the user to get accustomed to visually confirm the pattern.
To address the above problem, as shown in Patent Document 1, an apparatus that optically confirms liquid droplets ejected from nozzles so as to confirm an ejection state is installed in a wide format ink jet printer, etc., and it has been put into practice.    Patent Document 1: JP-A-2006-187981
Furthermore, in a conventional serial-type image forming apparatus, as shown in Patent Document 2, a device for detecting an ejection state is provided near an idle-ejection receiver in which liquid droplets not contributing to image formation are ejected.    Patent Document 2: JP-A-2005-319698
As opposed to this, in a line-type image forming apparatus, it is not necessary for a head unit to move during its printing operation. Therefore, Patent Document 3 describes a liquid ejection recording apparatus that has an ejection detecting device in a cap member of a maintenance and recovery apparatus that maintains and recovers a head unit and detects liquid droplets when the head unit moves to the position for a maintenance and recovery operation.    Patent Document 3: JP-B2-2838894
Furthermore, Patent Document 4 describes an ink jet printer that has a device for detecting an ejection state on a path on which a head unit moves to the position for a maintenance and recovery operation.    Patent Document 4: JP-A-2005-199658
Furthermore, Patent Document 5 describes an apparatus that has a moving unit that makes it possible for a detector for detecting an ejection state to be moved relative to a head unit.    Patent Document 5: JP-A-2006-192789
Furthermore, Patent Document 6 describes an ink jet apparatus that provides detectors for respective nozzles.    Patent Document 6: JP-A-2006-213051
As for the apparatuses described in Patent Documents 3 and 4 that move the head unit to the maintenance position, thereby making it possible to detect an ejection state, it is necessary to move a head position during its printing operation so as to confirm the ejection state. Therefore, in order to confirm the ejection state, printing speed may be remarkably reduced.
On the other hand, according to the configuration described in Patent Document 5 that has the moving unit that makes it possible for the detector to be moved relative to the head unit, it is difficult to precisely move the detector. In addition, if there is a failure in the moving unit of the detector, a target head unit cannot be detected. Therefore, sufficient reliability is not obtained.
Furthermore, according to the configuration described in Patent Document 6 that has the detectors for the nozzles, it is possible to confirm an ejection state during a printing operation, thus attaining high speed and reliability. However, because the detector is provided for each nozzle, the interval between the detectors is small. Therefore, it is necessary to provide the detectors with high processing precision. In addition, with the provision of many detectors, a wiring circuit becomes complicated. Therefore, the costs of the head unit increase due to an ejection detecting mechanism. Furthermore, because the number of the detectors correspond to the number of the nozzles, it is necessary to maintain all the detectors in a normal state. Therefore, it is difficult to maintain the reliability of the detectors themselves. Moreover, in the case of optical detection, heat is generated when light-emitting elements drive as the detectors, thereby causing the rise of the temperature of the head unit and the reduction of the sensitivity of light-receiving elements. Accordingly, when the detectors are provided for the respective nozzles, a lot of heat is generated due to the accumulation of the light-emitting parts.